Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 261739 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 139 PM EDT Wed Apr 26 2017 .SYNOPSIS... Low pressure along the Mid-Atlantic Coast will move northeast and away from the area today. High pressure will build across the southeastern United States resulting in a considerable boost to the temperature with above seasonal warmth expected by the weekend. An increase in southwesterly wind flow will bring enough moisture back into the area by late Thursday to promote widely scattered afternoon and evening showers and thundershowers over the mountains. && .NEAR TERM /THROUGH TONIGHT/... As of 1030 AM EDT Wednesday... Expecting a fair weather day with lots of sunshine. No major adjustments needed. Previous discussion... Let the warm-up begin. Our 5 days of drought busting rain has come to an end. The only lingering effect will be some morning cloudiness, otherwise looking for a return of the sun and much warmer temperatures. Afternoon highs today will be about 15 degrees warmer than yesterday... should hear a lot of lawn mowers going today. Still have two main Stem Rivers in flood. See the hydro section for details. Synopsis... Low pressure was centered just off the Mid-Atlantic coast. This feature is forecast to move northeast and away from the area today. An upper ridge of High pressure is forecast to amplify over the southeastern CONUS, while a vigorous upper level short wave trough ejects northeast through the Mississippi river valley. With increasing heights and warming thicknesses, 85h temperatures will climb to near +15 deg C by this afternoon. This surge of warmth will translate into surface temperatures in the 70s. With all the moisture in the ground, dewpoints will remain elevated (50s to near 60), so tonight`s low temperatures will be mild with lows only dropping into the 50s. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... As of 400 AM EDT Wednesday... The overarching highlight in the short-term period will be for a unseasonably warm, summerlike temperatures. Potential exists for showers and thunderstorms mainly along/west of the Blue Ridge heading into the latter part of the week, though confidence is low on timing. For Thursday: First part of the day starts out warm and starting to feel a bit more humid as strong sunshine works to evaporate the moisture left behind from the multiday rain event. We should also have a reasonably strong enough south to southwest breeze as low- level lapse rates get steeper with aforementioned insolation warming temperatures well into the 70s to a few lower 80s. Increasing 500 mb height falls toward the latter half of the day will help to erode what is modeled to be a strong capping inversion between 850-700 mb, ahead of a surface cold front. Model guidance supports some chance- level PoPs in western counties (from a line from Hot Springs to Roanoke to Reidsville west) generally during the late afternoon hours. The GFS is the earliest and feel this probably is a bit aggressive given the stout cap that needs to be overcome. With the cold front undergoing frontolysis as it advances eastward, I`m also not really that sold on frontal lift, and the mid-level trough deamplifies and lifts northeast generally west of the Appalachians. Feel that we`ll end up seeing a weakening line of showers and thunderstorms toward late afternoon and then weakening further into the mid-evening hours as it progresses into the VA/NC Piedmont. Kept lows a bit warmer than MOS in the mid 50s to mid 60s warmest in the Piedmont and coolest west of the Blue Ridge. Friday and Saturday feature the re-establishment of an amplified mid- level ridge across a large part of the Southeastern CONUS, well ahead of a powerful mid-level trough over the Four Corners Region. A developing, though very slow moving surface low will be located somewhere in the vicinity of the Texas Panhandle Friday and along the Red River in Oklahoma/Texas on Saturday. A warm front will extend northeastward from it along the Ohio Valley to perhaps as far east as the Eastern Continental Divide. It`s really during this period of time where global model guidance consistency begins to suffer from timing discrepancies on when any lift generated from the warm front may spark showers and thunderstorms. That is important because while the air mass in low-levels is relatively capped, progged instability aloft is substantial (-2 to -4 Showalter indices and MUCAPES in excess of 1000 J/kg), even into the evening hours. I could see potential for thunderstorms pretty much either day mainly from the Blue Ridge west, though it is conditional on lift associated with the warm front making its way into our area. As described, confidence is quite low on timing but wouldn`t rule out an isolated stronger storm or two if storms can develop. Kept a rather generic 25% PoP or less along and west of the Blue Ridge for each day as well as Friday evening until there`s better consistency. Should see highs upper 70s to mid 80s on Friday, rather mild lows in the upper 50s to mid 60s Friday night, and widespread 80s on Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... As of 300 AM EDT Wednesday... We will be in a very summerlike pattern this weekend with very warm and humid conditions and an increasing chance for showers and thunderstorms each day, especially along the Blue Ridge. By Monday, the vertically stacked upper closed low over the midwest will move into the Great Lakes region. This will push a vigorous cold front into the area from the west with a good chance of showers and thunderstorms through Monday afternoon. High pressure will then build on for Tuesday with cooler weather and windy conditions. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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As of 135 PM EDT Wednesday... Expecting VFR conditions all sites through the early portion of the TAF period. Then, decent radiational cooling will combine with ambient ground moisture from recent rainfall to generate some patchy fog overnight. Expect IFR/LIFR due to cigs/vsbys at KBCB and KLWB with possible tempo to similar conditions at KLYH and KDAN. Any fog/stratus should burn off tomorrow morning and effects of cold front approaching from the west look to hold off lower cigs and chances for thunder until after this valid period so essentially VFR conditions through about noon Thursday. Winds will become a bit gusty after daybreak. Extended Aviation Discussion... Better flying weather is expected for the second half of the week. Much warmer weather can be expected through the upcoming weekend with temperatures much above normal. Warming surface temperatures also mean increasing atmospheric instability. As such, potential will exist for cloud build-ups and the risk for isolated to scattered thunderstorms along the spine of the Appalachians during the peak heating hours each afternoon.
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&& .HYDROLOGY... As of 530 AM EDT Wednesday... No additional rainfall expected the next 24 hours. Two mainstem rivers are experiencing flooding within the RNK Hydrologic Service Area. The Lower Roanoke at Randolph, and the Dan River. The Dan River is by-far in worse shape of the two rivers with water levels into the major flood category downstream of Danville. The Dan River at Danville has crested...and the river will gradually recede upstream of Danville toward Wentworth. At Paces and South Boston, the situation is a bit more serious as the river at both locations is considered major flooding. At Paces the river is expected to crest this morning at between 28.5 and 29 feet. This is almost 9 feet above flood stage. At 30 feet water would reach the floor of the Highway 658 bridge. Viewing the crest history, this is only the 9th time of record that the river has crested above the major flood level of 27 ft at Paces. At South Boston the river will continue to rise today with a crest near 29 feet. This is 10 feet above the flood stage of 19 feet. At 29 feet, a number of businesses in the Riverdale area are affected. It is interesting to note that most of the previous major floods for the Dan River were associated with a tropical system, such as Hurricane Agnes in 1972 or Hurricane Fran in 1996. For additional details on the river flooding, please refer to the AHPS page and specific FLS/FLWs issued from this office. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PM NEAR TERM...MBS/PM SHORT TERM...AL LONG TERM...MBS AVIATION...MBS/PM HYDROLOGY...PM

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